Underground initiated miscible



United States Patent 3,126357 UNDERGRGUND lNiTlATED MHSCWLE BANK FLGQDEIG John C. McKinnell, Taft, alif., assignor to California Research Corporation, San Francisco, C'alif a corporation of Delaware No Drawing. Filed Feb. 17, W61, Ser. No. 89,918

8 tjlaims. ((Il. 166-11) This invention relates to the assisted recovery of petroleum, and more particularly this invention relates to the assisted recovery of petroleum by means of an underground initiated miscible bank followed by a liquid flood.

In most producing areas, oil is first recovered by making use of natural formation pressure to move oil toward producing wells through which it may be lifted to the surface. After the natural formation pressure has been depleted, various secondary recovery methods are utilized. It is in this general area that the present invention is concerned, although it is preferred to use the term assisted recovery which, while including secondary recovery as known in the art, also includes primary or tertiary, quaternary, etc. development of the formation.

The most widely used method of assisted recovery and generally the most economical is water flooding. However, even after a successful Water flood it is often found that forty to seventy percent of the original crude oil remains in the formation. Therefore, many other methods have been tried to improve petroleum recovery. Included among these methods are various techniques of underground combustion and miscible flooding.

Generally an underground combustion drive is accomplished by igniting a formation adjacent a borehole and then moving the combustion front through the formation by injecting an oxygen-containing gas through the borehole into the formation. Products of the drive are produced at various recovery boreholes as the combustion front passes through the formation. The theory is that an increase in temperature will aid oil recovery by lowering the viscosity of the oil in the formation thus making it move more readily to the producing wells.

Although in some instances underground combustion has proven successful in recovering large amounts of oil, there are many problems associated with an underground combustion drive. The tremendous cost of compressing and injecting air into the formation is one of the major drawbacks of the underground combustion process. These costs are a major factor in limiting the application of underground combustion processes to relatively shallow formations. in addition, there are many chemical and engineering diiiiculties inherent in the underground combustion drive. These difliculties are particularly troublesome as the combustion front progresses away from the injection borehole and the distance between the injection borehole and the combustion front becomes larger.

In miscible flooding generally a liquefiable hydrocarbon substance miscible with the formation oil is injected into the oil-bearing formation to set up a bank or zone of miscible products in the formation. This zone is then moved substantially through the producing formation to a producing well by injecting a driving fluid behind the miscible bank. The miscible agents conventionally employed in these floods are usually readily liquefiable low boiling point hydrocarbons such as propane. Ideally the propane or other hydrocarbon substance is injected through an injection borehole into the oil-bearing formation where it is miscible with the formation oil. However, many practical problems including the problems of obtaining the miscible agent and of injecting the agent into the formation and obtaining the desired miscibility with the formation oil have hindered miscible flooding.

3,126,957 Patented Mar. $1, 1964 Briefly, the present invention provides an improved method of assisting the recovery of petroleum which provides utilizing some of the desirable features of each of the above-discussed assisted recovery methods, and specifically the present invention provides for establishing a high temperature zone adjacent an injection borehole by means of combustion oil in the vicinity of the injec tion borehole for a limited distance into the oil-bearing formation, injecting crude oil through the injection borehole and into the high temperature zone where some cracking takes place causing a bank of miscible hydrocarbon end products to form on the perimeter of the high temperature zone, again injecting air into the formation through the injection borehole on the occurrence of a predetermined condition to reignite any coke or residuum products adjacent the borehole to re-establish the high temperature zone and then injecting addition oil through the injection borehole into the high temperature zone to increase the size of the bank of miscible products and after the miscible bank reaches a predetermined size, injecting water through the injection borehole to drive the miscible bank substantially through the formation while recovering products of the drive at recovery wells.

It is an object of the present invention to provide a relatively low-cost, high-recovery method of assisted recovery of petroleum.

In order to practice the present invention, combustion is first initiated in an oil-bearing formation. In some formations there will be enough residual oil in place adjacent the injection borehole so that the cornbustion may be initiated and supported using only this oil as a fuel. However, it is in keeping With the spirit of the invention to inject oil initially into the formation adjacent the injection borehole if there is insufficient residual oil to support combustion remaining adjacent the injection well, and to ignite this injected oil. Ignition may take place by any one of a number of suitable means. One suitable method is taught in the Simm patent US. 2,747,672. As therein disclosed, a solution of phosphorus in carbon disulphide is injected into the oil-bearing formation and is later followed by the injection of air to cause ignition of the phosphorus which in turn ignites the oil in the formation.

Combustion is continued in the vicinity of the injection well by continuing the injection of air. The combustion front should be moved out into the formation a limited distance. The distance the combustion front is moved into the formation of course will vary depending on the particular formation thickness and the particular arrangement of injection and producing wells. However, 10 feet in most formations should be considered the minimum distance to advance the combustion front from the injection borehole. The extent beyond the minimum desirable distance to establish the high temperature zone also depends on particular formation vari ables such as formation thickness, rate of heat lost and permeability, but in most cases 50 to feet should be considered the outer limit to extend the combustion front. When the combustion front has reached the predetermined limits, air injection is stopped, leaving the high temperature bank adjacent the injection well.

Crude oil is now injected through the injection well into the high temperature zone. The crude oil is preferably a low value crude oil which is available in the area. Although most gravity crudes will produce some of the desirable cracked products, it is preferred to inject crudes in the 10 to '20 AH gravity range. The oil is passed through the high temperature zone at a rate suitable to form low molecular weight, liquid cracked products which are more easily moved than the high molecular weight fractions of the crude oil. The cracked products move out into the cooler area in front of the U combustion zone. The injection of crude oil is continued until a predetermined condition occurs in the formation.

The predetermined condition may be either a drop in temperature below the level at which the desired cracking of the injected crude will take place under the formation pressure, or a predetermined increase in injection pressure of the crude oil. It is preferred to regulate crude oil injection on an injection pressure variable rather than a. temperature variable because of the difficulty of precisely measuring temperature in a producing formation." A series of thermocouples or other temperature-measuring devices could however be arranged in boreholes drilled on the perimeter of the high temperature zone to record the temperature. If temperature measurements are available, the injection of crude oil continues until the temperature of the hot zone falls below a value which will give the desired cracked products. This temperature will vary for different crudes and must be predetermined in a laboratory with miscibility tests on the particular formation oil and injected crude oils utilized in the process.

It is preferred however to program the injection of crude oil on the injection pressure required to inject the crude into the formation. This pressure may be read directly at the injection well. The particular pressure at which crude injection should be discontinued must be determined as a function of core permeability and the viscosity of the injected crude. As the crude is cracked, the heavier ends tend to deposit in the formation in the form of coke and oil residuum while leaving the formation substantially free of fluids. The pressure needed to force it through the high temperature zone will increase as these products reduce the size of the available flow channels. The preferred pressure increase to signal stopping crude injection is best determined by laboratory core tests on the particular formation to be flooded.

After oil injection is stopped, the high temperature zone adjacent the injection borehole may be re-established by igniting the coke and residuum oil remaining near the injection borehole. Reignition may often be accomplished by injecting air or an oxygen-containing gas into the formation to cause spontaneous combustion of the coke. If the formation will not ignite spontaneously, it may be ignited in any suitable manner such as taught in the Simrn patent US. 2,747,672.

The desired temperature range to maintain the high temperature zone adjacent the injection borehole varies depending on the injected crude and the particular molecular weight products which produce the desired miscibility with the formation oil. This temperature range may be established by tests on the particular formation and oils in question. However, in most formations a range of between 700 F. and 1200 F. gives the best result.

To maintain the predetermined desired temperature range in the formation adjacent the injection borehole, temperature control devices are necessary. One manner of controlling temperature in the formation is to alternate the injection of water or stream with the injection of air or crude oil to control the temperature and the size of the heat bank. Water or steam may precede oil injection to cool the formation if it is too hot. Alternatively, air may be injected to initiate combustion and thereby elevate the temperature of the high temperature zone.

\Vhen the desired miscible bank has been formed in the formation, water is injected through the injection borehole to move the miscible bank substantially through the oil-bearing formation. The optimum size of the miscible bank depends on the particular formation to flooded and the production well spacing. Rarely will the miscible bank exceed one-tenth of the pore volume of the formation. The water injection may follow, either an air or an oil cycle. The determining factor is the attainment of a miscible bank size suitable to be moved through the formation and recover a substantial portion of the oil therein. The Water flood is accomplished using standard equipment and engineering practices utilized in a conventional water flood.

When the miscible products begin to appear at the recovery well, it is sometimes desirable to shut in the recovery well. However, it is more often desirable to continue to produce the well and in some instances to recirculate the miscible products again through the producing formation by injection into a well around which it is advantageous to establish a miscible bank.

The invention as described provides a method of assisting the recovery of petroleum utilizing an undergroundgenerated miscible bank followed by a Water flood. The invention having been fully described, I claim:

1. The method of assisting the recovery of petroleum products from an underground formation which is penetrated by at least one injection well and one recovery well which comprises injecting air into said formation through said injection Well and initiating combustion in a localized portion of said formation adjacent said injection Well, continuing the injection of air until at least a portion of the combustion front has been moved away from the said injection well, discontinuing said air injection, injecting crude oil into the heated portion of said formation to cause at least a partial cracking of said crude oil and the formation of a bank of light hydrocarbon end products on the perimeter of the heated portion of said formation, continuing the injection of crude oil into the formation until the occurrence of a predeterminable condition indicates that substantial cracking of the injected crude oil has stopped, and then injecting water into said formation through said heated portion to transport said bank of hydrocarbon end products substantially through said formation to said recovery well and recovering petroleum products at said recovery well.

2. The method of claim 1 where the crude oil has a specific gravity in the range of from 10 to 20 API and the heated portion of the formation is maintained between 700 and 1200 F.

3. The method of assisting the recovery of petroleum products from an underground formation which is penetrated by at least one injection well and one recovery well which comprises injecting air into said formation through said injection well and initiating combustion in a localized portion of said formation adjacent said injection well, continuing the injection of air until at least a portion of the combustion front has been moved away from the said injection well, discontinuing said air injection, injecting crude oil into the heated portion of said formation to cause at least a partial cracking of said crude oil and the formation of a bank of hydrocarbon end products on the perimeter of the heated portion of said formation, continuing said injection of crude oil until a'predetermined condition is sensed in said formation indicating that substantial cracking of the injected crude oil has stopped, stopping said injection of said crude oil, again injecting air into said formation through said injection well and initiating combustion in said formation, continuing said injection of air until a similar portion of said formation has been exposed to combustion as when ignition was first started, and then injecting Water into said formation through said heated portion to transport crude oil, including the bank of hydrocarbon end products, to said recovery well and recovering petroleum products at said recovery well.

4. A method of assisting the recovery of petroleum from a petroleum-bearing formation penetrated by at least one injection well and one recovery well which comprises injecting air through said injection well into said formation and initiating combustion in a localized section of said formation adjacent the said injection well, continuing injection of air through said injection Well to move the combustion front a limited distance from said injection well leaving a substantial zone of the said formation adjacent said injection well at a high temperature and substantially free of fluids, discontinuing the injection of air through said injection well, injecting crude oil through said injection well into the high temperature zone of said formation at an injection rate to cause cracking of a portion of the injected crude oil in said high temperature Zone and the formation of a bank of hydrocarbon end products on the perimeter of said high temperature zone, continuing said injection of said crude oil until a predetermined condition is sensed in the said formation indicating that substantial cracking of the injected crude oil has stopped, discontinuing the injection of said crude oil through said injection well and again injecting air into the said formation through said injection well and reinitiating combustion in said formation adjacent said injection well to again raise the temperature of a substantial zone of said formation adjacent said injection well, stopping said air injection, injecting additional crude oil into said high temperature zone to cause cracking of a portion of the injected crude oil thereby adding hydrocarbon end products to said bank of hydrocarbon end products, alternately injecting crude oil and air into said formation in the manner above described for a plurality of injection cycles, and then at the end of a selected injection cycle injecting water into said formation to force the injected crude oil and said bank of hydrocarbon end products substantially through said formation toward said producing well and recovering products at said producing well.

5. The method in accordance with claim 4 wherein the said predetermined condition sensed in said formation is a predetermined increase in the injection pressure of the crude oil through said injection well.

6. The method in accordance with claim 4 wherein the said predetermined condition is a predetermined drop in temperature of the high temperature zone.

7. The method in accordance with claim 4 wherein after the injection of air through said injection well and before the injection of crude oil through said injection well water is injected through said injection Well to lower the temperature of the formation adjacent said injection well to below about 1200 F.

8. The method in accordance With claim 4 wherein after the injection of air through said injection well and before the injection of crude oil through said injection well steam is injected through said injection well to lower the temperature of the formation adjacent said injection well to below about 1200 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,423,674 Agren July 8, 1947 2,796,132 Bruce June 18, 1957 2,880,802 Carpenter Apr. 7, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,,l26 957 March 31, 1964 John C. McKinnell It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 2 line 7, for "combustion" read combusting column 3 line 59, for "stream" read steam line 69 after "to" insert be -o Signed and sealed this 11th day of August 1964.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. THE METHOD OF ASSISTING THE RECOVERY OF PETROLEUM PRODUCTS FROM AN UNDERGROUND FORMATION WHICH IS PENETRATED BY AT LEAST ONE INJECTION WELL AND ONE RECOVERY WELL WHICH COMPRISES INJECTING AIR INTO SAID FORMATION THROUGH SAID INJECTION WELL AND INITIATING COMBUSTION IN A LOCALIZED PORTION OF SAID FORMATION ADJACENT SAID INJECTION WELL, CONTINUING THE INJECTION OF AIR UNTIL AT LEAST A PORTION OF THE COMBUSTION FRONT HAS BEEN MOVED AWAY FROM THE SAID INJECTION WELL, DISCONTINUING SAID AIR INJECTION, INJECTING CRUDE OIL INTO THE HEATED PORTION OF SAID FORMATION TO CAUSE AT LEAST A PARTIAL CRACKING OF SAID CRUDE OIL AND THE FORMATION OF A BANK OF LIGHT HYDROCARBON END PRODUCTS ON THE PERIMETER OF THE HEATED PORTION OF SAID FORMATION, CONTINUING THE INJECTION OF CRUDE OIL INTO THE FORMATION UNTIL THE OCCURRENCE OF A PREDETERMINABLE CONDITION INDICATES THAT SUBSTANTIAL CRACKING OF THE INJECTED CRUDE OIL HAS STOPPED, AND THEN INJECTING WATER INTO SAID FORMATION THROUGH SAID HEATED PORTION TO TRANSPORT SAID BANK OF HYDROCARBON END PRODUCTS SUBSTANTIALLY THROUGH SAID FORMATION TO SAID RECOVERY WELL AND RECOVERING PETROLEUM PRODUCTS AT SAID RECOVERY WELL. 